Solve for $k$ : $k - 9 = 4$
Add $9$ to both sides: $ \begin{eqnarray} \\ k - 9 &=& 4 \\ \\ {+9} && {+9} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{-9 + k} &\hphantom{=}& \hphantom{4} \\ k &=& 4 {+ 9} \\ \end{eqnarray} $ Simplify. $ k = 13$